<p>This study systematically investigates the regulatory mechanism of damage in recycled aggregate concrete (RAC) and natural aggregate concrete (NAC) with varying levels of slag content (<i>S</i> = 0%-70%). The multiscale formation mechanism of strength is analyzed from the perspectives of microstructural evolution and mesoscopic damage by combining statistical damage theory and microscopic testing technology. The findings indicate that the peak stress of RAC experiences a onsistent decline with an increase in slag-equivalent replacements for cement (SERC), reaching a maximum decrease of 48.2%. Conversely, NAC demonstrates optimal mechanical properties at a slag content of 35%, exhibiting a 9.2% increase in peak stress. Scanning electron microscope (SEM), energy-dispersive X-ray spectroscopy (EDS), differential scanning calorimetry (DSC), and nuclear magnetic resonance (NMR) analysis indicate that moderate amounts of slag can refine the internal pores of concrete, promote C-S-H gel formation, and strengthen the interface transition zone (ITZ). However, it is demonstrated that excessive addition results in the aggregation of unreacted particles and the coarsening of pores. The correlation between macroscopic stress-strain behavior and mesoscopic damage mechanisms is established, and it is determined that the mechanism governing concrete strength development is jointly determined by microstructural mechanical characteristics and the evolution of mesoscopic damage during force-deformation. The differential regulation mechanism of slag admixture on the performance of concrete materials is revealed, providing a theoretical basis for the engineering application of RAC and the utilization of construction waste as a resource.</p>